A belt transmission device for transmitting the rotation of an engine crankshaft to various engine accessories includes, as shown in FIG. 20, a pulley arm 63 provided on the slack side of a belt 61 so as to be pivotable about a pivot shaft 62, a tension pulley 64 rotatably mounted on the pulley arm 63 at its end opposite to the pivot shaft 62, and a hydraulic auto-tensioner A for applying an adjusting force to the pulley arm 63, thereby biasing the pulley arm 63 such that the tension pulley 64 is pressed against the belt 61, to keep constant the tension of the belt 61.
JP Patent Publication 2000-504395A discloses a hydraulic auto-tensioner A used in a belt transmission device of this type. This hydraulic auto-tensioner includes a cylinder having a bottom and containing oil, a sleeve extending vertically from the inner bottom surface of the cylinder, a rod having its lower portion slidably inserted in the sleeve, thus defining a pressure chamber in the sleeve, a spring seat provided at the upper portion of the rod, and a return spring mounted between the spring seat and the inner bottom surface of the cylinder and biasing the rod and the cylinder in the direction in which the rod protrudes from the cylinder.
This auto-tensioner further includes a bellows having its respective ends fitted on the outer periphery of the spring seat and the outer periphery of the cylinder at its upper portion, defining a closed reservoir chamber between the cylinder and the sleeve. The reservoir chamber has its lower portion in communication with the pressure chamber through an oil passage in which a check valve is provided. When a pushing force is applied to the hydraulic auto-tensioner A from the belt 61 through the tension pulley 64 and the pulley arm 63 that tends to push the rod into the cylinder, the check valve is adapted to close, thus causing oil in the pressure chamber to flow through a minute leakage gap defined between the radially inner surface of the sleeve and the radially outer surface of the rod. The pushing force is thus damped by hydraulic damping force produced in the pressure chamber due to viscous resistance of the oil flowing through the minute gap.
In this conventional arrangement, when a pushing force is applied to the rod from the belt 61, the pushing force pushes oil in the pressure chamber into the minute gap defined between the radially inner surface of the sleeve and the radially outer surface of the rod. The pushing force is thus damped by the hydraulic damping force produced in the pressure chamber due to viscous resistance of oil flowing through the leakage gap. The hydraulic damping force is thus substantially proportional to the pushing force. This means that the hydraulic damping force increases with an increase in the pushing force.
Thus, this tensioner cannot prevent over-tension of the belt and the resulting deterioration in durability of the belt.
To prevent this problem, JP Patent Publication 2009-191863A proposes to mount a relief valve in an oil passage defined in the rod through which the pressure chamber communicates with the reservoir chamber such that the relief valve opens if the pressure in the pressure chamber exceeds a set pressure, thereby releasing oil in the pressure chamber into the reservoir chamber through the oil passage. With this arrangement, the pressure in the pressure chamber never exceeds the set pressure of the relief valve, which prevents over-tension of the belt.
In the arrangement of the hydraulic auto-tensioner disclosed in JP Patent Publication 2009-191863A, if the relief valve opens, oil in the pressure chamber is instantly released into the reservoir chamber through the oil passage, resulting in a sudden drop in pressure in the pressure chamber. This makes it difficult for the auto-tensioner to reliably perform its hydraulic damping function.
When assembling the belt transmission device for driving engine accessories shown in FIG. 20, after mounting the hydraulic auto-tensioner in position and after mounting the tension pulley 64 on the pulley arm 63, if the hydraulic auto-tensioner disclosed in JP Patent Publication 2009-191863A is used in the belt transmission device of FIG. 20, since the rod and sleeve are biased by the return spring in the direction in which the rod protrudes from the sleeve, it is necessary to push the rod into the sleeve against the force of the return spring before stretching the belt around the tension pulley. Thus, it is difficult and troublesome to assemble the belt transmission device.
Typically, the oil used in a hydraulic auto-tensioner to produce hydraulic damping force contains a synthetic oil of polyalphaolefin and ester as its base oil. Such oil has a relatively high viscosity with a kinematic viscosity of 90 to 100 mm2/second at 40° C. and 16 to 18 mm2/second at 100° C.
If such oil is used in a hydraulic auto-tensioner, in order to keep small the damping load, the leakage gap is ordinarily adjusted to 20 to 60 μm, and the stroke of the valve ball for opening and closing the valve hole of the check valve is adjusted to about 0.2 to 0.3 mm.
In the hydraulic auto-tensioner disclosed in JP Patent Publication 2009-191863A, too, as with other ordinary hydraulic auto-tensioners, if relatively high-viscosity oil is used, and the leakage gap is adjusted to 20 to 60 μm to keep small the damping load, the stroke of the auto-tensioner tends to be large, so that a large amount of oil flows out into the reservoir chamber from the pressure chamber when the rod is pushed in.
If the stroke of the check ball between the open and closed positions is adjusted to 0.2 to 0.3 mm, the same amount of oil that has flowed out of the pressure chamber cannot smoothly flow back into the pressure chamber from the reservoir chamber when the rod protrudes, so that the amount of oil in the pressure chamber decreases. Especially at a low temperature, since the oil viscosity increases further, the amount of oil that flows into the pressure chamber decreases further, thus markedly reducing the damping load.